Variable speed AC induction motor controller

Abstract

A circuit for powering a two-phase AC induction motor. The circuit generates a first signal of the form Vdc+A sin(2πft-0°) and a second signal of the form Vdc+A sin(2πft-90°). The first signal is input to a first error amplifier along with a first sampled difference signal from the motor. The second signal is input to a second error amplifier along with a second sampled difference signal from the motor. The outputs from each of the first and second amplifiers is input into a first comparator and a second comparator along with a sawtooth waveform to create a first sinusoidally modulated square wave signal and a second sinusoidally modulated square wave signal. The first and second sinusoidally modulated square wave signals are fed to driver circuits which in turn control an H-bridge circuit for powering the motor from a DC bus.

What is claimed is: 
     
       1. A circuit for controlling power to a two-phase induction motor, the circuit comprising: a. first means for generating a first signal having the form Vdc+A sin(2πft-0°);   b. second means for generating a second signal having the form Vdc+A sin(2πft+90°) coupled to the first means for generating:   c. means for forming a third signal by pulse-width modulating a fourth signal representative of a difference between a first difference between two motor winding voltages and the first signal;   d. means for forming a fifth signal by pulse-width modulating a sixth signal representative of a difference between a second difference between two motor winding voltages and the second signal; and   e. means for driving the motor coupled to receive the third signal and the fifth signal for generating six motor drive control signals.   
     
     
       2. The circuit for controlling power according to claim 1 wherein the means for driving the motor comprises: a. a multiplexor coupled to receive the third signal and the fifth signal for forming a first multiplexed signal and a second multiplexed signal;   b. a first plurality of drivers coupled to receive the first multiplexed signal; and   c. a second plurality of drivers coupled to receive the second multiplexed signal.   
     
     
       3. The circuit for controlling power according to claim 2 further comprising a circuit for powering two windings in the motor comprising an H-bridge arrangement coupled to the first plurality of drivers and coupled to the second plurality of drivers for providing power to windings of the motor. 
     
     
       4. The circuit for controlling power according to claim 1 wherein the first means and the second means comprise a multiplexor coupled to control a direction of motor rotation and further wherein the means for driving the motor comprises: a. a first plurality of driver coupled to receive the third signal; and   b. a second plurality of drivers coupled to receive the fifth signal.   
     
     
       5. The circuit for controlling power according to claim 1 wherein an amplitude of the motor winding voltages are controlled depending upon a slip angle. 
     
     
       6. A circuit for controlling power to a two-phase AC motor, the circuit comprising: a. a first generator circuit for forming a first sinusoidal signal wherein the first sinusoidal signal has first phase angle;   b. a second generator circuit for forming a second sinusoidal signal wherein the second sinusoidal signal has a second phase angle and further wherein the first phase angle differs from the second phase angle by ninety degrees;   c. a first sampling circuit coupled to receive a first winding voltage of the motor and a second winding voltage of the motor for forming a first difference signal;   d. a second sampling circuit coupled to receive the second winding voltage of the motor and a third winding voltage of the motor for forming a second difference signal;   e. a first amplifier circuit coupled to compare the first sinusoidal signal to the first difference signal;   f. a second amplifier circuit coupled to compare the second sinusoidal signal to the second difference signal;   g. a third generator circuit forming a periodic signal coupled to a third amplifier circuit wherein the third amplifier circuit is coupled to compare the periodic signal to the first difference signal for forming a first PWM signal;   h. a fourth amplifier circuit coupled to compare the periodic signal to the second difference signal for forming a second PWM signal; and   i. a driver circuit coupled to receive the first PWM signal and the second PWM signal for controlling a first winding current and a second winding current.   
     
     
       7. The circuit for controlling power according to claim 6 wherein the first winding voltage and the second winding voltage are each controlled depending upon a slip angle. 
     
     
       8. A method of controlling power to a two-phase induction motor comprising the steps of: a. forming a first signal having a sinusoidal waveform;   b. forming a second signal having a sinusoidal waveform wherein the second signal is 90 degrees apart in phase from the first signal;   c. forming a third signal by pulse-width modulating a fourth signal representative of a difference between a first difference between two motor winding voltages and the first signal;   d. forming a fifth signal by pulse-width modulating a sixth signal representative of a difference between a second difference between two motor winding voltages and the second signal; and   e. generating six motor drive control signals for driving the motor based upon the third signal and the fifth signal.   
     
     
       9. The method according to claim 8 further comprising the step of controlling an amplitude of the motor winding voltages depending upon a slip angle. 
     
     
       10. The method according to claim 8 wherein the step of generating the six motor drive control signals comprises: a. forming a seventh signal and an eighth signal by multiplexing the third signal and the fifth signal;   b. generating a first plurality of motor drive control signals based upon the seventh signal;   c. generating a second plurality of motor drive control signals based upon the eighth signal.   
     
     
       11. The method according to claim 10 further comprising the step of providing power to windings of the motor using an H-bridge arrangement. 
     
     
       12. The method according to claim 8 further comprising the step of interchanging the first signal and the second signal depending upon a logic level of an input to a multiplexor and further wherein the step of generating the six motor drive control signals comprises: a. generating a first plurality of motor drive control signals based upon the third signal; and   b. generating a second plurality of motor drive control signals based upon the fifth signal.   
     
     
       13. The method according to claim 12 further comprising the step of providing power to windings of the motor using an H-bridge arrangement. 
     
     
       14. A method of controlling power to a two-phase induction motor comprising the steps of: a. forming a first pair of sinusoidal signals 90 degrees apart in phase;   b. forming a second pair of sinusoidal signals representative of differences between motor voltage signals;   c. forming a third pair of pulse width modulated signals representative of differences between the first pair of signals and the second pair of signals; and   d. driving the motor with a driver circuit wherein the driver circuit is coupled to receive the third pair of signals.   
     
     
       15. The method according to claim 14 further comprising the step of adjusting the motor voltage signals depending upon a slip angle. 
     
     
       16. The method according to claim 14 further comprising the step of interchanging the signals of the first pair depending upon a desired direction of motor rotation. 
     
     
       17. The method according to claim 14 wherein the step of driver the motor with a driver circuit includes providing power to windings of the motor using an H-bridge arrangement. 
     
     
       18. A circuit for forming phase shifted waveforms comprising: a. means for forming a first triangle waveform wherein the first triangle waveform is centered about a reference voltage level;   b. means for forming a second triangle waveform coupled to the means for forming the first triangle waveform, the means for forming the second triangle waveform comprising means for charging a first capacitor with a current wherein the current flows in a first direction when the first triangle waveform is greater than the reference voltage level and wherein the current flows in a second direction when the first triangle waveform is less than the reference voltage level.   
     
     
       19. The circuit according to claim 18 wherein the first triangle waveform has a first phase angle and the second triangle waveform has a second phase angle and further wherein the first phase angle and the second phase angle differ by ninety degrees. 
     
     
       20. The circuit according to claim 18 further comprising means for controlling a frequency of the first triangle waveform. 
     
     
       21. The circuit according to claim 18 wherein the means for forming the second triangle waveform further comprises means for charging the first capacitor to the reference voltage level when the first triangle waveform reaches a peak voltage level. 
     
     
       22. The circuit according to claim 21 wherein the means for forming the first triangle waveform comprises: a. a first current source coupled to charge a second capacitor when a first switch is in a first position;   b. a second current source coupled to discharge the second capacitor when the first switch is in a second position;   c. a hysteresis comparator having a first hysteresis comparator input, a second hysteresis comparator input and a hysteresis comparator output wherein the first hysteresis comparator input is coupled to the second capacitor, the second hysteresis comparator input is coupled to the reference voltage level, and wherein the hysteresis comparator output is coupled to control the first switch.   
     
     
       23. The circuit according to claim 22 wherein the first current source and the second current are variable for controlling a frequency of the first triangle waveform. 
     
     
       24. The circuit according to claim 22 wherein the means for setting the second triangle waveform to the reference voltage level comprises: a. a switch control circuit for forming a switch control signal coupled to receive the hysteresis comparator output wherein the switch control signal attains a first switch control signal level for a predetermined period of time before attaining a second switch control signal level when the first triangle waveform reaches the peak voltage level; and   b. means for coupling the reference voltage level to the first capacitor when the switch control signal attains the first switch control signal level.   
     
     
       25. A circuit for driving an AC motor having a plurality of windings, the circuit comprising: a. means for receiving a first input signal; and   b. a feedback loop for forming a first differential drive signal across a first motor winding wherein the first differential drive signal is related to the first input signal by a predetermined gain.   c. means for receiving a second input signal; and   d. a second feedback loop for forming a second differential drive signal across a second motor winding wherein the second differential drive signal is related to the second input signal by the predetermined gain wherein the first input signal has a first period and the second input signal has a second period wherein the first period is equal to the second period and further wherein the first input signal and the second input differ in phase by 90 degrees.   
     
     
       26. A circuit for controlling voltage to an AC motor comprising: a. first means for forming a signal representative of a desired slip angle coupled to receive a signal representative of a motor voltage;   b. second means for forming a signal representative of an actual slip angle coupled to receive the signal representative of a motor voltage and coupled to receive a signal representative of a motor current; and   c. means for controlling the motor voltage coupled to the first means for forming and coupled to the second means for forming.   
     
     
       27. The circuit according to claim 26 wherein the first means for forming comprises: a. means for forming a delayed signal wherein the delayed signal and the signal representative of the motor voltage have a predetermined phase difference;   b. a logic gate having a first logic gate input, a second logic gate input and a logic gate output, wherein the first logic gate input is coupled to receive the signal representative of the motor voltage and the second logic gate input is coupled to receive the delayed signal and further wherein the logic gate output provides the signal representative of the desired slip angle.   
     
     
       28. The circuit according to claim 26 wherein the second means for forming comprises a logic gate having a first logic gate input, a second logic gate input and a logic gate output, wherein the first logic gate input is coupled to receive the signal representative of the motor voltage and the second logic gate input is coupled to receive the motor current and further wherein the logic gate output provide the signal representative of the actual slip angle. 
     
     
       29. The circuit according to claim 26 wherein the means for controlling comprises: a. means for charging a capacitor when the signal representative of the desired slip angle is a logical high voltage and the signal representative of the actual slip angle is a logical high voltage.   b. means for discharging the capacitor when the signal representative of the actual slip angle is a logical low voltage; and   c. means for maintaining a charge on the capacitor when the signal representative of the desired slip angle is a logical low voltage; wherein the capacitor is coupled to control the motor voltage.     
     
     
       30. A circuit for driving an AC motor comprising: a. means for receiving a first input signal; and   b. a first feedback loop for forming a first differential drive signal across a first winding of the motor wherein the feedback loop forces the first differential drive signal to be related to the first input signal by a predetermined gain.   
     
     
       31. The circuit according to claim 30 wherein the differential drive signal is a feedback signal. 
     
     
       32. The circuit according to claim 30 wherein the input signal is substantially a sinusoid. 
     
     
       33. The circuit according to claim 30 wherein pulse width modulation is employed for forming the first differential drive signal. 
     
     
       34. The circuit according to claim 30 further comprising: a. means for receiving a second input signal; and   b. a second feedback loop for forming a second differential drive signal across a second winding of the motor wherein the second differential drive signal is related to the second input signal by the predetermined gain.